The next time you're holding a dollar bill, it might be worth looking further than the big number one - you could get more bang from your buck than you expected.

Ordinary low denomination bills are raking in thousands of dollars in an online trend that is centred on the eight-digit serial number which appears on each U.S. banknote.

'Unusual' bills are being bought, sold and hunted on the website CoolSerialNumbers.com, with low serial numbers, from 00000001 to 00000100, being particularly sought after, a $1 bill with the serial number 00000002 going for $2,500.

Mark up: A $1 bill with the serial number F00000001 C would be worth thousands of dollars

The U.S.-based site lists all the different notes that collectors are looking out for and allows serial number fans to get in touch with one another.

There are categories such as 'solids' (where the digits repeat eight times), 'ladders' (12345678), 'radars' (01133110 - where the number reads the same left-to-right as right-to-left) and 'repeaters' (20012001 - the second half is the same as the first half).

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Then there are 'radar repeaters' (12211221), 'super radars' (20000002 - all the internal digits are the same) and 'super repeaters' (where the first two digits are repeated four times, such as 63636363).

Big bucks: It may not be worth $99,999,999 dollars - but the serial number means this note is likely to go for a bit more than $5

Prize money: 'Repeaters' and 'super repeaters are also highly regarded on the website CoolSerialNumbers.com

High stakes: If 'trailing nine' of 19999999 is worth $750 on the website, imagine what this prize specimen must have gone for

Dave Undis, the a Nashville musician and currency collector who runs the site, is happy to include some criteria that might go unnoticed to the untrained eye - such as a 'pi note' with the number 31415927.

A $5 bill with the number 33333333 is currently up for sale for $13,000, while a set of nine $20 bills running from 00000010 through to 00000090 can be bought for $1,800.

A Google+ link on the site allows you to scroll through a full catalogue of unusual serial numbers.

The site gets about 5,000 visitors a year, according to Undis, who says he has been collecting currency with 'cool serial numbers' for about 30 years.

The U.S. Government introduced numbers onto banknotes in 1928, and has always used eight digits.

HIGH-SELLING SERIAL NUMBERS

$1 E 99999995 A $1,500

$1 B 00000002 J $2,500

$1 P 00000004 E $1,250

$1 B 00000004 J $1,000

$1 S 00000003 D $1,000

Set of four notes $5,00099999993-99999996

Source: CoolSerialNumbers.com

So it is particularly unusual to find a banknote without a serial number - and the site regularly sells error notes without serial numbers.

Collectors have long been interested in rare and flawed money - and not just in the U.S.

In September last year, the Bank of England auctioned £5, £10, £20 and £50 notes with unusual serial numbers - such as those coming at the beginning or the end of a run, often characterised by the prefix A01.

In 2009, a batch of 20p pieces that had been minted without the date mark were valued at £50 - with one eBay seller appearing to make £7,100 from the sale of just one coin.

And it's not just about money, either - Yo-Yos.net is, as the name suggests, an online community for collectors of Yo-Yos with unusual serial numbers.